SPECTROSCOPIC STUDY OF CONFORMATIONAL-CHANGES IN SUBDOMAIN-1 OF G-ACTIN - INFLUENCE OF DIVALENT-CATIONS

Temperature dependence of the fluorescence intensity and anisotropy de cay of iodoacetyl)-N'-(5-sulfo-1-naphthyl)ethylenediamine attached to Cys(374) of actin monomer was investigated to characterize conformatio nal differences between Ca-and Mg-G-actin. The fluorescence lifetime i s longer in Mg-G-actin than that in Ca-G-actin in the temperature rang e of 5-34 degrees C. The width of the lifetime distribution is smaller by 30% in Mg-saturated actin monomer at 5 degrees C, and the differen ce becomes negligible above 30 degrees C. The semiangle of the cone wi thin which the fluorophore can rotate is larger in Ca-G-actin at all t emperatures. Electron paramagnetic resonance measurements on maleimide spin-labeled (on Cys(374)) monomer actin gave evidence that exchange of Ca2+ for Mg2+ induced a rapid decrease in the mobility of the label immediately after the addition of Mg2+. These results suggest that th e C-terminal region of the monomer becomes more rigid as a result of t he replacement of Ca2+ by Mg2+. The change can be related to the diffe rence between the polymerization abilities of the two forms of G-actin .